Optical coherence tomography (OCT) is an interferometric imaging technique with widespread applications in ophthalmology, cardiology, gastroenterology and other fields. In interferometric imaging, light from a known and controlled optical path (the ‘reference path’) is caused to interfere with light returned from an unknown path such that information about this unknown path (the ‘sample path’) may be determined by an analysis of the resulting interferogram. The interferogram contains the depth location information of structures within the sample being analyzed. A particular advantage of OCT is its inherent compatibility with fiber optics making it a nearly ideal imaging modality for non-invasive or minimally invasive medical procedures.
In general, for OCT systems, the lengths of the sample and reference paths are matched to ensure the interference effect being recorded corresponds to a desired scan region within the sample. In the case of relatively long optical catheters required in many procedures (approximately 1.5 to 2 meters is common) such matching can be difficult to achieve. Furthermore, the optical fibers used in these catheters can easily stretch or contract several millimeters during use.
When using OCT, the optical ‘zero-point’ is critical. This defines where, in the image space, the so-called reference plane exists. By convention, surface planes are in the x-y plane, and the depth occurs along the z-axis. In a microscope application for example, it may be beneficial to set the zero point at the surface of the microscope slide, so specimens can be measured against this known surface. For a catheter inserted in a lumen such as a blood vessel, the most useful reference plane is the outer surface of the catheter tip itself, and all distances are measured outward from this location.
OCT systems typically use an adjustable reference path within the optical imaging equipment to adjust to each catheter as it is used. This is generally handled using a reference motor which can move a reflector such as a reference mirror back and forth to adjust the reference path.
A given medical application may use many disposable catheters per day; all interfaced to the same imaging equipment. Thus, while the primary path length adjustment can work quite effectively, it usually requires an initial adjustment by a skilled operator who understands the optical reflection pattern or ‘signature’ of the catheters that will be recorded by OCT to determine how to adjust the reference path to coincide with the outer surface of the catheter tip.
Again, the adjustment of the image zero-point, or reference plane location is performed by adjusting the primary path-length of the reference arm. This adjustment is often termed ‘z-offset’ of the reference arm and is controlled via a motor, called simply the z-offset motor, and a movable reference mirror. By convention, the instrument z-offset is zero when the sample arm length (catheter) is manufactured exactly as designed; is negative when the catheter is too short; and positive when the catheter is too long. Motor movements can be used to adjust the reference path in a consistent manner for different catheters.
OCT catheter-based probes typically include a beam directing structure such as lens or reflector placed at their distal tip to focus and direct light for scanning purposes. The light typically propagates through one or more transparent sheaths that comprise the catheter outer structure with an optical fiber disposed therein and in optical communication with the lens or reflector. Each of the optical interfaces can cause a reflection that will be detected by OCT. Hence, it may be challenging to determine which of those reflections corresponds to the desired optical reference point (‘zero-point’) of the system.
Since measurements are made based on this zero-point setting, setting it correctly can significantly affect the results of a given medical application. Furthermore, because there may be several closely spaced and similar intensity reflections, the use of software to detect the proper zero-offset (‘z-offset’) is problematic and unreliable. Additionally, as a further complexity, because imaging systems and the disposable catheter-based based probes such systems use change over time, software for one system is generally not designed for different OCT probes. Calibration drift and other imaging artifacts can also affect image quality during a review of frames post-pullback.
Therefore, methods, devices and systems suitable for calibrating an OCT system are needed. Given the complexity of optical interface signals, additional techniques, software modules, and devices to address such signals in the context of calibrating a data collection probe or the underlying data are needed. In addition, image processing techniques suitable for dealing with calibration drift and related issues after a pullback is complete are also needed. Further, the methods, devices, and systems should be able to work with different types of disposable data collection probes. The present invention addresses these needs and others.